1. Technical Field
The present invention relates to a novel method of producing estolides, in which the advantageous properties of existing estolides are retained and the shortcomings thereof are solved, and more particularly, to a method of producing estolides having high structural stability.
2. Description of the Related Art
Petroleum resources, which significantly contribute to the pollution of the global environment, are strictly regulated at refineries. Moreover, petroleum resources obtained by raising S, N, heavy metals, aromatics, etc. up to the earth's surface from deep underground are refined to thereby produce lubricants, but such lubricants have low biodegradability. When introduced to ecosystems, such lubricants have great influences on biological circulatory systems, and problems of ecological disturbance caused by chemicals may occur in the real world. Furthermore, direct crude oil spills, chemical spills, and silent oil spills may occur, causing problems for ecosystems.
Thus, efforts to replace petroleum resources with environmentally friendly alternatives are being made. A representative material thereof is biomass-derived oil.
Biomass-derived oil is a primary product obtained by squeezing fruits resulting from planting and cultivating trees. Since carbon from such biomass-derived oil comes from CO2 in the atmosphere of the earth, unlike mineral oil derived from fossil carbon, there is no additional generation of CO2 in the atmosphere of the earth, and the CO2 concentration in the atmosphere may in fact decrease, thus contributing to total CO2 reduction through environmental rehabilitation. Consequently, the use of biomass-derived oil positively affects environmental rehabilitation, and may contribute to total CO2 reduction through environmental rehabilitation, thereby allowing for the additional use of fossil carbon.
Although diesel has been used as a drilling fluid, at present only 100% environmentally friendly chemicals are permitted to be utilized based on legal regulations due to awareness of the problem of such environmental issues. As for diesel, biomass-derived environmentally friendly diesel is regulated to be compulsorily used in amounts less than 10% in most of the world, and the proportion thereof is gradually increasing.
As for lubricating oil, legal regulations for the use of environmentally friendly lubricating oil have not yet been introduced. Although ester lube has been introduced as an environmentally friendly lubricating oil, it is a chemical and is thus about four times as expensive as crude oil-based lubricating oil, and the production amount thereof is limited, making it impossible to produce the amounts required by markets. However, there is already consensus that currently useful lubricating oil is an environmental pollutant. Lubricating oil resulting from crude oil is known to have biodegradability of 10 to 30% on the basis of CEC analysis, and 1 liter of lubricating oil prepared from crude oil is already regarded as an environmental pollutant that pollutes 1 million liters of water. However, as techniques for manufacturing biomass-derived lubricating oil have been recently devised, the use of environmentally friendly lubricating oil has come to the fore. For example, regulations governing the use of environmentally friendly oil at places adjacent to oceans, rivers and the like have been passed in December 2013 in the USA, and a bill (California senate bill 916) stipulating that 25% of gasoline and diesel engine oil be replaced with environmentally friendly lubricating oil by 2017 has been proposed in California. Although the bill in California failed to be established due to the high cost of manufacturing environmentally friendly lubricating oil, the fundamental purport thereof resonates with most people, and there is a continuous need for the use of environmentally friendly lubricating oil.
In order to manufacture environmentally friendly lubricating oil having high biodegradability and containing no toxic materials (S, N, aromatics, heavy metals), methods of using biomass may be readily taken into consideration. Since biomass is an environmentally friendly material having a very high biodegradability of about 70 to 100% and does not contain toxic materials such as S, N, aromatics and heavy metals, when lubricating oil is manufactured using biomass, environmentally friendly lubricating oil as described above may be expected to result. Also, petroleum resources are problematic because CO2, which is a greenhouse gas, is added to the earth's circulatory system, whereas biomass is a hydrocarbon that is already present in the earth's circulatory system. Hence, the additional conversion of biomass into lubricating oil means that the hydrocarbons of biomass are simply converted into lubricating oil in terms of the earth's overall circulatory system, advantageously preventing the additional generation of CO2 in the earth's circulatory system. The existing biomass industry has problems such as the generation of only small amounts of biomass and the requirement to collect biomass, but biomass commercialization markets are becoming very large, and thus crude palm oil (CPO), soy bean oil (SBO) and the like may be traded in amounts of at least 1 million tons on open markets in Singapore and Indonesia. Furthermore, byproducts such as free fatty acids may be purchased in amounts of hundreds of thousands of tons on open markets, and may then be made into products, thus eliminating problems regarding the amounts and collection of materials.
These days, estolide is receiving great attention as a biomass-derived environmentally friendly lubricating oil. If bill 916 of California had been actually established, estolide was intended to be used as an environmentally friendly lubricating oil. The term ‘estolide’ refers to any material in which unsaturated double bonds of hydrocarbons are crosslinked with a carboxyl functional group. Estolide, which is naturally present in castor- or Lesquerella-derived vegetable oil, was noted as being simply synthesized by Penoyer et al in 1954, and thus shows promise as a novel product.
The applicability of estolide as lubricating oil (Group V, Ester base oil) was initially recognized due to the structural properties thereof. For example, triglyceride-derived estolide, which was prepared in the beginning, exhibits a good pour point (PP 9 to −36° C.) but has poor oxidation stability (RPVOT 29 to 52 min), and thus cannot be directly used as lubricating oil. As techniques have been devised for improving oxidation stability using oleic acid as an estolide feed through partial hydrogenation by use of an additive, the applicability thereof to high-quality lubricating base oil and cosmetic materials is significantly increasing.
A conventional process for producing estolide includes four steps of de-esterification, estolide synthesis, esterification, and hydrogenation. De-esterification is a step of converting triglycerides, which constitute most biomass-derived oil, into fatty acids, estolide synthesis is a step of converting unsaturated fatty acids into estolides, esterification is a step of converting the carboxyl functional group of an estolide into ester through a reaction with alcohol so as to stabilize it, and hydrogenation is a step of eliminating unsaturated double bonds from an estolide to thereby increase oxidation stability.
The estolide thus produced manifests characteristics of high-quality lubricating base oil having high viscosity index, oxidation stability and thermal stability, compared to conventional petroleum-based Group I, Group II, and Group III base oil products. Estolides are considerably favorable in terms of making lubricating base oil having high viscosity based on 100 vis.
However, conventional methods of producing estolides have the following problems.
The first problem is the dependence on oleic acid. Early estolide research was ongoing into the direct preparation of estolides from triglycerides so as to be used as lubricating base oil. However, in the case where a triglyceride is directly used, low-temperature stability may become problematic, and thus the resulting oil is unsuitable for use as lubricating base oil. Hence, as oleic acid is selectively used to produce estolide, the problem of low-temperature stability may be significantly alleviated, and the properties may be enhanced. In other words, the dependence on oleic acid is remarkably increased when producing estolide. However, the amount of biomass-derived oleic acid is limited. Table 1 below shows the hydrocarbon chains that constitute the triglycerides of CPO and SBO, which are commercially applicable. As is apparent form Table 1, oleic acid comprises about 52 wt % of palm oil. The remaining materials other than oleic acid are not contained in estolides, and are thus not used. Only the amount of biomass-derived oil corresponding to oleic acid may be used, which is merely 50 wt % at most. The remaining fatty acids other than oleic acid are disadvantageous in that there is no end use therefor.
TABLE 1Fatty acidSBOPO12:0 Lauric acid<1.214:0 Myristic acid 0.40.5 to 5.914:1 Myristoleic acid16:0 Palmitic acid 7 to 1432 to 5916:1 Palmitoleic acid<0.5<0.618:0 Stearic acid1.4 to 5.51.5 to 8.018:1 Oleic acid19 to 3027 to 5218:2 Linoleic acid44 to 625.0 to 14 18:3 Linolenic acid4.0 to 11 <1.520:0 Eicosanoic acid<1.0<1.022:0 Docosanoic acid<1.0
Second, alcohol is essentially required for esterification. Due to the presence of the fatty acid functional group, a variety of problems related to material stability and corrosion may occur in the estolide reaction, and thus such a functional group has to be converted into some other stable form. Typically, it is converted into ester, which has high stability and from which a volume gain may be expected. In order to convert estolide into ester, an acid functional group is conventionally reacted with alcohol, and is thus stabilized in the form of ester. In other words, alcohol is necessarily required in order to achieve reaction stabilization. Since alcohol is not produced during the reaction, it has to be introduced from the outside.
Third, hydrotreating is essentially required. In conventional estolide production reactions, hydrofinishing is performed in order to remove unsaturated double bonds from biomass-derived oil. Since oxidation stability is decreased in the presence of unsaturated double bonds, such unsaturated double bonds must be essentially removed through hydrogenation. In the conventional estolide reaction, unsaturated double bonds of estolides are eliminated through hydrogenation, especially hydrofinishing. However, hydrogenation is problematic because it requires reaction conditions of high temperature and high pressure and also because the price of hydrogen is very high, undesirably negating economic benefits. Hence, the production of estolides without conducting hydrogenation is regarded as very desirable.
Fourth, there may remain unsaturated double bonds in estolides, despite the reaction for removing unsaturated double bonds using such hydrogenation. In the case where lubricating oil has unsaturated double bonds in the molecular structure thereof, there may occur side reactions, including discoloration through the coupling of unsaturated double bonds and oxygen in air, and the high likelihood of corrosion due to high bindability with moisture in the air. Accordingly, it is important that unsaturated double bonds are completely removed through hydrogenation so that no unsaturated double bonds remain. For estolides, some ester bonds may break in the course of the reaction for completely removing unsaturated double bonds, and thus selective removal of unsaturated double bonds is carried out under the condition that ester bonding is maintained. For this reason, it is difficult to completely remove unsaturated double bonds. Unsaturated double bonds may be left behind at a level of less than 10 based on the iodine value.
Fifth, existing estolide has an ester functional group having low steric hindrance. Esterification is advantageous because the unique structural stability of ester may be expected, and also a volume gain due to alcohol may be expected. Although the ester functional group merely has relatively high stability compared to other functional groups, it is not absolutely stable. Depending on the reaction conditions, the ester functional group may be irreversibly converted into fatty acid. In this case, there may occur temporary but serious problems of aggravating the corrosion of engines. As for fatty acid methyl ester (FAME), which is a first-generation ester-type biodiesel, or for ester base oil, which is a Group V base oil, problems of the corrosion of engines attributable to fatty acids resulting from breaking the ester functional group are still reported. With the goal of overcoming such problems, another type of diesel or anti-corrosion additive may be used together therewith.